1. Field of the Invention
This invention relates to new macrocyclic polyether compositions and to complexation of ionic metal compounds therewith.
The novel crown ether compositions of this invention have been given the cognomen "lariat ethers". They have been designed to contain a macrocyclic polyether ring as found in simple crown ethers, but are substituted by a sidearm bearing a neutral Lewis basic donor group such as oxygen or nitrogen. The aforementioned donor group must be at a distance from the macroring such that a metallic cation complexed in the ring will be within bonding (solvating) distance of the donor group. Whereas non-sidearm containing crown ethers whose hole sizes are similar to the cation diameters complex cations by enveloping them in a two-dimensional matrix of donor groups, the sidearm of the lariat ether can reach over and solvate from above, adding a three-dimensional component of solvation to the complex. The concept of roping and tying an animal with a lasso suggests the name "lariat ether" since the ring binds the cation and further stabilization is provided by the sidearm donor group. The name "lariat" comes from the Spanish, la reata=the rope.
2. Description of the Prior Art
The complexation of metallic cations by macrocyclic (crown) polyethers was first demonstrated by Pedersen in 1967 (J. Amer. Chem. Soc., 1967, 87, 7017). Since his original disclosure literally thousands of ether compositions have been prepared. The remarkable structural diversity is apparent from the recent compilation of crown ether structures [G. W. Gokel and S. H. Korzeniowski, Macrocyclic Polyether Syntheses, Springer Verlag, Berlin, 1982 ].
Although crown ethers had been prepared by Luttringhaus and Ziegler (Justus Liebigs Ann. Chem., 1937, 528, 155) about three decades before Pedersen reported his first survey, there was little interest in these molecules. The reason for this is that it was not recognized before Pedersen's reports that the crowns could bind (complex) alkali metal and alkaline earth cations.
The crowns are typically large ring (macrocyclic) compounds characterized by repeating ethyleneoxy (--CH.sub.2 --CH.sub.2 --O--) units. The ability to bind metallic cations in either polar or nonpolar solutions derives from three aspects of the crown's structure. First, there is a hydrophilic or polar cavity which consists of several oxygen atoms. The oxygen electron pairs combine to provide an electron rich coordinating cavity which binds the cation. Second, since the cavity is circular and the metallic cations spherical, there is a complementary relationship between the two. In fact, for any given crown ether, there is a maximum in binding strength when the crown's hole size and the cation diameter are most similar (G. A. Melson, (Ed.) Coordination Chemistry of Macrocyclic Compounds, Plenum, N.Y., 1980, p. 145). Third, the exterior of the crown is hydrocarbon-like (lipophilic) and can be solvated by a nonpolar solvent. This configuration of hydrophilic hole and hydrophobic skin provides a solvent gradient within a single molecule. The binding of sodium cation by 15-crown-5 is illustrated in equation 1 below: ##STR1##
The crown complexes are all highly dynamic (R. M. Izatt and J. J. Christensen, Eds., Synthetic Multidentate Macrocyclic Compounds, Academic Press, N.Y. 1978, p. 245). The net binding of a cation is given by the equilibrium stability constant, Ks, which is simply the ratio of complexation (k.sub.c) and the decomplexation (k.sub.d) rates, i.e., Ks=k.sub.c /k.sub.d. The forward or complexation rates are typically near the diffusion-controlled limit, and the magnitudes are about 10.sup.8 M.sup.-1 sec.sup.-1. Decomplexation rates are also high but must be lower than the complexation rate if Ks is to be greater than unity.
An important point about most crown ether complexes is that the relationship of the cation to the ligand is essentially two-dimensional. That is, the crown wraps about the cation in essentially a plane of donor groups. Since the cation is itself spherical, it seems reasonable to assume that a spherical solvator would be a stronger binder. The cryptands (J. M. Lehn, Acc. Chem. Res., 1978, 11, 49) are very much stronger binders than crowns, and these materials do, in fact, offer the cation a three-dimensional array of binding sites.
A number of compounds having macrorings and sidearms have previously been reported. Notable among these examples are the work of Montanari (Tetrahedron Letters, 1979, 5550) and Woo (U.S. Pat. No. 4,256,859). These reports deal with crown compounds bearing sidearms designed to assist in appending the crown ether to a polymer matrix or backbone. Numerous others have reported similar efforts (see, for example, Okahara et al, J. Org. Chem., 1980, 45, 5355). The common feature of all of the compounds described in these reports is that the arm is designed either to enhance the lipophilicity of the crown or to allow for attaching the macroring to a polymer. In no case is any additional binding anticipated from or imputed to these substituted crowns by virtue of sidearm donor interaction.